The chemical used for etching of the resistive film to form individual resistors in a thin film thermal print head is extremely hard to control, particularly after it has etched through the resistive film. Such a process is referred to in allowed U.S. Patent Application Ser. No. 524,108 entitled "Thin Film Thermal Print Head", filed on Nov. 15, 1974 by Frank Ura. In that process, etching continues into the glass glaze, actually eroding the mesa beneath portions of the resistor. Since those portions of the resistor are left unsupported they tend to crack or break off even with addition of a wear layer material at the end of the fabrication process. The overall reliability of the thermal print head is thereby substantially diminished.
To preclude etchant from attacking the glass glaze material, a layer of etchant-resistant material is applied to the entire area of glass glaze including the resistor mesas prior to depositing the resistive material. The etchant-resistant layer should be chemically inert to the etchant used, thus protecting the glass glaze from attack by the etchant.